Recently, from the viewpoint of prevention of global warming, energy-saving is strongly demanded, and energy-saving is an urgent problem in electric household appliances. Especially in warm and cold storage container, a thermal insulator having an excellent adiabatic performance is required from the viewpoint of efficient use of heat.
Generally, thermal insulators include fibrous material such as glass wool, and foamed body such as urethane foam. To enhance the adiabatic performance of these thermal insulators, it is required to increase the thickness of insulator. By increasing the insulator thickness, however, the size is limited in the space to be filled, and it is difficult to save the space or utilize the space effectively.
Accordingly, a vacuum insulator is proposed as insulator of high performance. This is a thermal insulator made from a core member having a spacer function, being inserted in an enveloping member having gas barrier performance, and sealed by evacuating the inside.
By raising the degree of vacuum in the vacuum insulator, an adiabatic effect of high performance is obtained. The gas existing in the vacuum insulator is roughly classified into three types. First is a residual gas not exhausted at the time of manufacture of vacuum insulator. Second is the gas generated from the core material or enveloping member after vacuum sealing, such as gas adsorbed in core material or enveloping member, or reaction gas generated by reaction of unreacting components of core material. Third is the gas invading from outside through the enveloping member.
To adsorb these gases, the vacuum insulator may be filled with adsorbent. For example, carbon dioxide or moisture in the vacuum insulator is adsorbed by general adsorbent such as silica alumina (see, for example, Japanese Patent Application Laid-Open No. 61-103090, or cited reference 1).
The gas in the vacuum insulator is also adsorbed by Ba—Li alloy (see, for example, Japanese Patent Application National Publication No. 9-512088, or cited reference 2). Of the gases to be adsorbed by the adsorbent in the vacuum insulator, one of the most difficult gases to be adsorbed is nitrogen. This is because the nitrogen molecule is a nonpolar molecule having a large bonding energy of about 940 kJ/mol, and it is hard to activate. By using Ba—Li alloy, however, nitrogen can be adsorbed, and the degree of vacuum can be maintained within the vacuum insulator.
An oxygen absorbent is made of iron powder, oxidation promoting substance, filler, and moisture donor (see, for example, Japanese Patent No. 3252866, or cited reference 3).
This is an oxygen absorbent used in quality maintaining application of food and medicine, and moisture is needed for oxygen absorption.
In cited reference 1, gas of low activity such as nitrogen is adsorbed, and it is hard to realize. In cited reference 2, nitrogen can be adsorbed at ordinary temperature without requiring heat treatment for activation, but higher activity and larger capacity are demanded, and since Ba is a toxic substance, and sufficient safety for environment and human heath is required for industrial use. Beside, the alloy must be melted, and a huge energy is needed in manufacture. In cited reference 3, moisture is needed for oxygen absorption, and it is hard to use in an atmosphere not allowing even a trace of moisture.